Murdock: Make them blink in sequence.
A couple of weeks ago we tried building a blinking light interface using 3 LED's and a push button. The aim was to introduce a greater understanding of circuits, some more complicated wiring, and to get started on some basic code. We also learnt about resistors, which are a major component of most Arduino projects and ensure that our other parts don't fry.
It was a difficult session with some people excelling at the coding and others the wiring. And everyone gained invaluable experience with one of the foundations of electronics: troubleshooting! Whilst not everyone got their interface working, it certainly laid the foundations for building the robot building we are working on now.